1. Field of the Invention
The present application relates to a current-limiting device and a light-emitting diode (LED) apparatus containing the same. The current-limiting device includes a plurality of positive temperature coefficient (PTC) devices and is connected to LED in series for real-time current regulation.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Like a diode, current flows through an LED with forward bias and will be cut off with a reverse bias. However, the LED current flowing therethrough emits light, and an operating voltage for forward conduction is usually higher than that of a diode such as 0.7 volts. The operating voltage does not change as current varies. Thus the LED cannot directly connect to a power source with a voltage higher than the forward voltage and a current-limiting resistor is utilized to prevent the LED from burn-out caused by high current. In a case that a battery of 9V is directly connected to two ends of the LED and the LED has a forward voltage of 2.3V, the LED attempts to sustain a voltage of 2.3V at its two ends. However, the battery attempts to drive the voltage to 9V. Therefore, a voltage difference between 9V and 2.3V will cause a very large current to burn the LED out if there is only a very small resistance.
A simple resolution is to associate with a current-limiting resistor. FIG. 1A shows an LED circuitry 10, in which a current-limiting resistor 11 is connected to an LED 12 in series and coupled to a power supply voltage Vcc. The resistance R of the current-limiting resistor 11 can be obtained according to the equation: R=(Vcc−Vf)/If, where Vcc is power supply voltage, Vf is LED forward voltage, and If is LED operating current.
FIG. 1B exemplifies an LED I-V curve, in which Vcc=6V, Vf=3.8V and If=50 mA. The resistance of the current-limiting resistor 11 is 44Ω ((6V−3.8V)/0.05 A=44Ω). The current-limiting resistor is traditionally a fixed resistor, so that when Vcc increases to above 9V, Vf becomes greater than 4.2V. This event will induce that LED 12 heats up and be damaged.
The current-limiting resistor can provide LED protection as mentioned. However, because of high heat generation, a high power LED for illumination cannot instantly regulate its current when LED temperature rises. As a result, LED temperature will continuously increase or maintain at a high temperature, this event may burn the LED out or decrease the lifetime of the LED.